Aims
of PiGMaP
The genetic complement (or genome) of the pig is comprised of 18
pairs of chromosomes plus the X and Y (sex) chromosomes. Each chromosome
is made up of a long sequence of DNA base pairs and in total the
genome is comprised of about three billion DNA base pairs which
together define all the characteristics of the pig. The purpose
of the Pig Gene Mapping Project (PiGMaP) is to find genetic markers
(sequences of DNA bases which mark specific positions in the genome)
which are evenly spaced and cover the whole of the pig's genome
to produce a map on two levels: a genetic map and a physical map.
The genetic map shows the distances between markers estimated from
the amount of recombination that occurs between them in experimental
crosses (markers which are close together on the genetic map will
tend to be inherited together). The total genetic map length in
the pig is expected to be similar to that in man, i.e. about 30
Morgans (M), where one Morgan represents a distance in which on
average one genetic crossover occurs each time a gamete is formed.
PiGMaP aims to select markers that are spaced approximately 0.2
Morgans apart, so ultimately around 150 markers will need to be
chosen from the many that are screened. With enough markers, we
would expect to find a group of genetically linked markers representing
each chromosome. It will not be possible to say which linkage group
represents which chromosome without a physical map. On a physical
map some of the markers are located on individual chromosomes and
these provide landmarks which locate and orientate linkage groups
(Figure 1). It is intended that the project will result in one landmark
locus being found on each chromosome arm. Markers will be in the
same order on both the genetic and physical maps, but the relative
distances between markers will differ, principally because genetic
cross overs, and thus recombination, are more frequent in some chromosomal
regions than in others.
Uses
of a genome map
The markers on a genome map are valuable because they have two or
more different genetic variants (or alleles) present in the population
(such markers are said to be polymorphic). Polymorphic markers can
help to identify the position of any gene which causes appreciable
differences between individuals. This is because if the gene of
interest and a marker are close together (and hence genetically
linked) they will tend to be inherited together. Within a family
a particular marker allele may thus be found to be associated with
the trait of interest, which means that a gene for that trait must
be close to the marker. Genes responsible for human diseases, such
as cystic fibrosis, have been located in this way, with increasing
precision of localisation ultimately allowing isolation of the gene
responsible by `positional' cloning.
Most of the genes of agricultural importance (and many controlling
susceptibility to human diseases) do not have a big enough effect
on their own to produce large, qualitative differences between individuals.
Instead, variation in several or many genes of smaller effect combines
to produce continuous or quantitative variation in a trait between
animals. Thus such genes have been called quantitative trait loci
or QTL. An animal with genetically high growth rate, for example,
might have `high growth rate' alleles at a number of different QTL
throughout the genome. Except for a few special cases in experimental
organisms, geneticists have had to treat such quantitative variation
as a `black box', with no knowledge of how many QTL cause the variation,
their chromosomal locations or of their individual actions and interactions.
A genetic map will allow variation in economically important traits
to be dissected into variation caused by individual QTL. In a breeding
population variation for a QTL at the genetic level will cause variation
between animals at the physical level. If a QTL for a particular
trait is closely linked to a marker different marker, alleles will
appear to be associated with different levels of performance for
the trait. This association can be detected by statistical techniques
such as regression or maximum likelihood. If a complete genetic
map is available and sufficient animals are analysed, any QTL with
an appreciable effect on performance can be located between a pair
of linked markers. The positions of QTL in the genome can thus be
studied, as can the size and type of their effect and the importance
of interactions between QTL in the control of trait variation.
Once mapped, QTL could be manipulated by `marker assisted selection'
in which markers flanking the known position of a QTL are used to
select for increased frequency of the allele of value. Marker assisted
selection is likely to be of value for traits which can only be
measured in one sex, such as milk yield or litter size, and traits
with a low heritability. This is because these traits are more difficult
to improve by traditional selection and markers provide a means
estimating genetic potential, even in animals which are not expressing
the trait. Marker assisted selection may be particularly useful
for moving a few QTL alleles of value from one line or breed to
another. This can be done by selecting alleles for the markers flanking
the QTL from one breed and the remainder of the marker alleles from
the second breed. For example, it would be desirable to move the
genes for high litter size from the Chinese Meishan pig (see below)
to European breeds in this way.
Ultimately it may be possible to actually isolate (or clone) QTL
which have been mapped for detailed functional study. The approach
used to isolate the cystic fibrosis locus (i.e. positional cloning)
is laborious and could not currently be contemplated for porcine
QTL. In man, however, other loci responsible for genetic diseases
have been identified because a gene which had already been cloned
mapped to the same region as the disease gene. On subsequent study
this candidate locus has been confirmed to be the disease gene.
The same approach is feasible in the pig. It is known that the human
and mouse genetic maps are similar at the fine scale; the mouse
genome is like a human genome in which the chromosomes have been
cut up and the pieces reordered. What is more, individual genes
within a region are very similar (in terms of their function and
DNA sequence) in different mammals. As the porcine map is developed
it will be aligned with the human map, allowing study of comparative
mammalian genomic organisation. It is likely that the porcine map
will be very similar to the human map. Thus the continuing isolation
and characterisation of genes in man will provide a plentiful supply
of potential candidate genes for loci which have been mapped in
the pig. In other words, when a locus has been mapped in the pig,
we will know where the same locus is likely to be on the human map
and it will be possible to look through the database of sequenced
human genes in this region to identify potential candidates for
the gene in the pig.
The alignment of the human and porcine maps will also allow further
animal models for human diseases to be developed in the pig. An
example is porcine stress susceptibility, which is an animal model
of human malignant hyperthermia (Figure 1) and which allows the
causes and treatment of this disease to be studied in experimental
animals. Last but not least the knowledge of the molecular basis
of a disease might be used to establish a quick and economical means
of genetic screening of animals used for breeding. For example,
the ultimate goal of mapping the porcine stress susceptibility gene
is to enable the eradication of the disease by identifying and eliminating
carrier animals from the breeding stock and a test enabling this
has recently been developed.
Areas
of research: Reference populations
Pigs are particularly amenable for mapping purposes. In addition
to the relative ease with which they can be karyotyped, their short
generation interval (one year or less) and large litter size (around
10 in European breeds) means that informative reference populations
can be developed rapidly. As with other species, it is possible
to assess the genotype of a pig using the DNA from a small blood
sample obtained without harming the animal in any way.
A further major advantage of pigs lies in the genetic resources
available. The Meishan pig (Figure 2), recently imported from China
to three of the participating laboratories, is genetically very
different from European breeds such as the Large White (Figure 3),
whilst the European Wild Boar (Figure 4) is different from either
of these breeds. This genetic differentiation means that markers
may be fixed for different alleles in the two breeds (Figure 5)
and such markers are highly informative in the F2 population produced
from a cross between the breeds because the breed from which a particular
marker allele came can easily be identified.
As important as the breed difference for markers is the fact that
the breeds are phenotypically very different. The Meishan reaches
puberty at about 3.5 months (half the age at which the Large White
reaches puberty), produces a litter of about three to four more
piglets than the Large White and has a very placid nature, but is
slow growing with a fat carcass. Not only will QTL controlling these
differences be segregating and hence can potentially be mapped in
an F2 cross (Figure 6), but also each breed contains genes of value
to the other. Litter size in particular has proven difficult to
improve by traditional selection and is just the sort of trait of
low heritability, expressed in only one sex, for which marker assisted
selection may be of value. Thus not only will a genetic map allow
any genes with an appreciable effect on litter size to be identified,
it may be possible to rapidly select them into the Large White genetic
background, away from the deleterious effects of the Meishan genetic
background on growth rate and leanness.
Reference populations from a Meishan by Large White cross are
being produced in the UK and in France and the Netherlands. Other
participants in Germany and Sweden are using diverse crosses between
European commercial pigs and the Wild Boar as reference populations.
The exchange of DNA and genetic probes will allow the genetic
information from these populations to be brought together to produce
a single map.
Areas
of research: Genetic mapping
The ideal genetic marker is polymorphic (within limits, the more
alleles the better), codominant (i.e. both alleles can be distinguished
in all individuals) and easily scored. Markers based on DNA sequence
variation fulfil these criteria. Restriction fragment length polymorphisms
(RFLPs) caused by DNA sequence variation at a restriction site usually
have only two alleles and may not be very polymorphic within a breed.
In the cross between two divergent lines with very different allele
frequencies RFLPs can be very informative, however, because most
or all individuals in the F1 are heterozygous and the breed of origin
of alleles in the F2 is readily determined. One type of marker being
used in PiGMaP uses cloned porcine or human cDNA sequences as probes
to detect RFLPs (Figure 5). This type of marker is particularly
valuable in providing a skeleton of landmark loci, as the sequences
used have been mapped in humans and in the mouse. Thus potential
probes can be selected which cover the whole human map and so are
likely to be similarly distributed in the pig and mapping these
markers allows the porcine map to be directly aligned with that
of these other species.
Variable number tandem repeat (VNTR) loci provide another type of
marker based on DNA sequence variation. These markers have a variable
number of copies of a tandemly repeated `core' DNA sequence and
can be extremely polymorphic with many alleles and are found in
a wide range of species. Where the repeated core sequence is of
a ten or more base pairs VNTR loci are also known as `minisatellites'.
VNTR markers are very useful for individual identification and parentage
testing (e.g. by DNA fingerprinting using probes which detect several
or many minisatellite loci at the same time) and for linkage analysis
and genome mapping (using probes which each detect a single VNTR
locus). Because VNTR markers are so polymorphic they are not only
useful in crosses between breeds but within breeds as well. An example
of a single locus minisatellite banding pattern in a large pig sibship
which forms part of the PiGMaP reference population is shown in
figure 7. A more recently described type of VNTR marker is based
on a `microsatellite' core sequence, with several or many repeats
of two to four base pairs. These microsatellite markers are ideal
for mapping purposes because not only can they be highly polymorphic,
but their total length including flanking DNA is short enough (100
to 300 base pairs) to make them amenable to polymerase chain reaction
(PCR) amplification. In this technique only very small DNA samples
are required and the length variation between alleles can be rapidly
visualised on an acrylamide gel. Furthermore, it is now known that
there are many thousands of microsatellite loci distributed throughout
the genomes of many species, including pigs. Microsatellite loci
are being rapidly isolated within PiGMaP and those already characterised
are proving to be highly polymorphic and thus informative.
The PiGMaP skeleton map based on RFLPs detected with cDNA probes
will be fleshed out with very variable VNTR markers which will be
informative both between and within populations. The initial objective
is to produce a genetic map with markers approximately evenly spaced
every 0.2 Morgans, with one or two landmark loci physically mapped
on every chromosome arm. This will require around 150 marker loci
in total, although perhaps twice this number will have to be screened
to achieve this aim as some will be rejected for technical reasons
or for being closely linked to more informative markers.
Areas
of research: Physical mapping
The location of linkage groups on specific chromosomes can be achieved
in several ways. Cell lines which are hybrids between the pig and
the Chinese hamster or the mouse may contain only one or a few chromosomes
from the pig with the remainder from the other species (Figure 8).
Under the right conditions, hybridisation of porcine DNA sequences
across a panel of such lines, each with a different complement of
porcine chromosomes, can be used to identify the porcine genes which
are on the same chromosome. However, such cell lines are difficult
to characterise and the porcine component is not stable. An alternative
is to make use of the wide range of sizes and morphologies of porcine
chromosomes. This diversity raises the possibility of FACS (fluorescence
activated cell sorter) sorting of porcine chromosomes. The development
of a `flow sorted' karyotype within PiGMaP would enable the assignment
of DNA sequences to chromosomes, particularly as the PCR technique
allows the rapid assessment of whether a particular DNA sequence
is present in a small sample of chromosomes. The technology would
also allow other advances to be made, such as the development of
genomic libraries of DNA sequences from a single chromosome, allowing
the isolation of chromosome specific markers. Two of the research
groups within PiGMaP have already achieved the FACS sorting of porcine
chromosomes and are currently working to fully characterise the
flow karyotype (Figure 9).
In situ hybridisation allows the physical mapping of DNA sequences
to regions within a chromosome. In this technique, a radioactively
labelled DNA probe is hybridised to a metaphase chromosomal spread
and an autoradiograph of the result produced. The probe will hybridise
to complementary sequences on individual chromosomes, hybridisation
being indicated by silver grains on an autoradiograph (Figure 10).
This technique is currently laborious, but can be improved by the
use of fluorescent, rather than radioactive, labels (Figure 11).
Industrial and other support
PiGMaP has received strong support from virtually the whole of the
European pig breeding industry. In addition the project has been
welcomed by scientists working on the human and other mammalian
genomes as being complementary to their own research and for its
potential merit in providing animal models of human disease. Several
fruitful collaborative links between PiGMaP participants and human
geneticists have resulted.
Interest in the results of the research and its potential commercial
exploitation have been expressed by the following companies and
breeding organisations:
Bovar BV, Rosmalen, The Netherlands
Cobiporc,
Saint Gilles, France
Cofok,
Oosterhout, The Netherlands
Conseil
Regional de Bretagne, Rennes, France
Coopagri,
Landerneau, France
Cotswold
Pig Development Company Ltd., Rothwell, UK
Danske
Slagterier, Copenhagen, Denmark
Euribrid
BV, Boxmeer, The Netherlands
Fomeva
BV, Cuyk, The Netherlands
France
Hybrides, Evry, France
Gen'Ouest,
Ancenis, France
Institut
Technique du Porc, Paris, France
Masterbreeders
(Livestock Development) Ltd., Tring, UK
Meat
and Livestock Commission, Milton Keynes, UK
National
Pig Development Company Ltd., Driffield, UK
Newsham
Hybrid Pigs Ltd., Malton, UK
Nederlands
Varkens Stamboek NVS, Nijmegen, The Netherlands
Nieuw
Dalland BV, Venray, The Netherlands
Pig
Improvement Company Ltd., Fyfield Wick, UK
SCA
Pen Ar Lan, Maxent, France
Ucagenof,
Anvin, France
UCAAB,
Chateau Thierry, France
UPB
Porcofram plc, Ipswich, UK
Versele
Laga NV, Deinze, Belgium
Zentralverband
der Deutschen Schweineproduktion, Bonn, Germany
Progress
and the benefits of close collaboration
Work on PiGMaP has only just started in earnest. Each laboratory
is focussing on a few areas with unnecessary duplication being minimised,
but the spread of the project is large, from isolation of genetic
markers to biometrical genetic analysis and from FACS chromosome
sorting to reference family production (Table 1). With work underway
in 16 laboratories in eight European countries being coordinated
in a single EC BRIDGE project, rapid progress is already being made.
The reference families are nearing completion and DNA from founder
animals is being distributed to participants. Approximately 40 informative
RFLP markers have been identified with homologous and heterologous
probes, 10 locus specific VNTR markers have been characterised and
100 microsatellite loci have been sequenced and are being characterised.
Genes have been assigned to 13 of the 18 autosomes. Panels of hybrid
cell lines have been established and are being evaluated. Flow sorting
of porcine chromosomes has been established and the identification
of individual chromosomes is underway. A genomic library of chromosome
1 is being evaluated. Progress in the first six months of the project
has been such that it is anticipated that the PiGMaP target of a
porcine map produced and aligned with the human map within the three
years of the project will be achieved.
A project the magnitude and complexity of PiGMaP could not be contemplated
by a single laboratory or country. The EC BRIDGE programme has provided
an incentive and a framework for collaboration between laboratories
with complementary skills which makes the aims of PiGMaP achievable.
Furthermore, in making the project achievable, EC funding makes
it worthwhile for individual nations and industries to provide support
to the project, thus further increasing the rate of progress. Examples
of the cooperation and collaboration involved are agreements on
the range of DNA probes to be utilised by each laboratory, the physical
mapping by one laboratory of probes isolated by another, the agreement
to work on common reference families and to jointly analyse and
collate the data. A further major benefit of the project has been
the close and friendly links that have developed between all the
participants, with concurrent exchange of ideas, methods and technology.
At its initiation PiGMaP was the only European collaborative project
for genome mapping in farm animal species and the worth of the project
has been recognised by offers of collaboration and cooperation that
have been received from laboratories world wide.
Joint
Scientific Publications*
Archibald, A., Haley, C.S., Andersson, L., Bosma, A.A., Davies,
W., Fredholm, M., Geldermann, H., Gellin, J., Groenen, M., Gustavsson,
I., Ollivier, L., Tucker, E.M. and Van de Weghe. 1990. A. PiGMaP:
An European initiative to map the porcine genome. Anim. Genet.
22, Suppl. 1. 82-83.
Chowdhary, B.P., Johansson, M., Chowdhary, R., Ellegren, H., Gu,
F., Andersson, L. and Gustavsson, I. 1991. In situ hybridization
mapping and RFLP analysis of the porcine albumin (ALB) and transferrin
(TF) genes. Cytogenet. Cell Genet. (submitted).
Frengen, E., Davies, W., Kran, S., Thomsen, P., Kristensen, T. and
Miller, R. 1991. Specific amplification of porcine DNA from pig/rodent
hybrid cell lines using the polymerase chain reaction and primers
from a porcine short interspersed element. Anim. Genet. 22,
Suppl. 1. 88-89.
Frengen, E., Thomsen, P., Kristensen, T., Kran, S., Miller, R. and
Davies, W. 1991. Porcine SINEs: Characterization and use in species
specific amplification. Genomics 10: 949-956.
Haley, C.S., Archibald, A., Andersson, L., Bosma, A.A., Davies,
W., Fredholm, M., Geldermann, H., Groenen, M., Gustavsson, I., Ollivier,
L., Tucker, E.M. and Van de Weghe, A. 1990. The pig gene mapping
project: PiGMaP. Proceedings of the 4th World Congress on Genetics
Applied to Livestock Production, Edinburgh. XIII, 67-70.
Harbitz, I., Chowdhary, B., Chowdhary, R., Kran, S., Frengen, E.,
Gustavsson, I. and Davies, W. 1990. Isolation, characterisation
and chromosomal assignment of a partial cDNA for porcine 6 phosphogluconate
dehydrogenase. Hereditas 112: 83-88.
Harbitz, I., Chowdhary, B., Thomsen, P.D., Davies, W., Kaufmann,
U., Kran, S., Gustavsson, I., Christensen, K. and Hauge, J.G. 1990.
Assignment of the porcine calcium release channel gene, a candidate
for the malignant hyperthermia locus, to the 6p11>q21 segment
of chromosome 6. Genomics 8: 243-248.
Mariani, P., Johansson, M., Ellegren, H., Harbitz, I., Kumar Juneja,
R. and Andersson, L. 1992. Multiple RFLPs in the porcine calcium
release channel gene (CRC): assignment to the HAL linkage group.
Animal Genetics (in press).
Thomsen, P.B., Bosma, A.A., Kaufmann, U. and Harbitz, I. 1991, Preferential
loss of the porcine 6 phosphogluconate dehydrogenase gene in pig
x rodent somatic cell hybrids. Hereditas (in press).
Yerle, M., Archibald, A.L., Dalens, M. and Gellin, J. 1990. Localization
of the PGD and TGF beta 1 loci to pig chromosome 6q. Animal Genetics
21: 411-417.
Yerle, M., Dalens, M., Galman, O., Lahbib Mansais, Y., Archibald,
A.L. and Gellin, J. 1991. Localization on pig chromosome 6 of five
markers: GPI, APOE, TGFg1, ENO1 and PGD, carried by human chromosomes
1 and 19, using in situ hybridization. Anim. Genet. 22,
Suppl. 1. 81.
*It must be emphasized that the joint articles listed above had
appeared when the programme had been in existence for only 6 months.
It will need substantial updating by the time the programme ends.
Many additional publications have been produced individually from
the teams.
Glossary
Allele A variant form of a gene. For some genes two or more alleles
may be present in a population (see polymorphism).
cDNA Complementary DNA. DNA sequences which are derived from expressed
genes and are thus largely coding regions.
Candidate locus A gene whose characteristics (e.g. protein produced)
suggest that it may be involved in the control of the trait of interest.
Clone Copy of a specific DNA sequence (from the pig, man or another
species) usually maintained in a viral or bacterial host.
Coding region A sequence of DNA that encodes the sequence of part
of a protein.
F2 cross A cross between two pure lines or breeds produces an F1
population, crossing F1 animals with each other produces an F2 population.
Animals in the F2 population carry the range of genetic variation
from within and between the lines originally crossed
FACS Fluorescent Activated Cell Sorter. A machine capable of sorting
mixtures of cells and even individual chromosomes.
Flow karyotype Karyotype as defined by a FAC sort of chromosomes.
Genomic library Collection of clones each containing a sequence
of DNA. The whole library contains most or all of the DNA sequences
from the genome of an animal.
Genotype The genetic composition of an individual in terms of the
alleles it possesses.
Heterozygote An individual which has two different alleles at a
particular locus (such individuals are heterozygous).
Homozygote An individual which has two alleles which are the same
at a particular locus (such individuals are homozygous).
Hybrid cell line Cell line created by fusing cells from two different
species. Such a cell line often loses chromosomes from one of the
species and hence may ultimately contain only a few different chromosomes
from that species, the remainder coming from the second species.
In situ hybridisation Technique in which a labelled DNA sequence
is hybridised to complementary DNA sequences on a spread of chromosomes.
This allows the position of the DNA sequence on the chromosome to
be seen under the microscope.
Karyotype Description of the genetic complement of an animal in
terms of its number of chromosomes and their sizes and morphologies.
Linkage Association between two or more genes which are close together
on a chromosome and thus which tend not to be separated by recombination
(see also synteny). Such genes are said to be linked.
Locus The position of a gene in the genome.
Marker assisted Technique in which selection of the best animals
for breeding purposes is
selection aided by the marker genotype.
Minisatellite Type of VNTR locus at which there are several or many
tandem repeats of a DNA sequence of moderate length. Such loci are
often highly polymorphic with many alleles. The repeated (or core)
sequence is often found at several or many loci in the genome.
Microsatellite Type of VNTR locus. Similar to a minisatellite except
that the tandem repeats are of a short DNA sequence (often 2 base
pairs). Such loci are often highly polymorphic with many alleles.
The repeated sequence is often found at several or many loci in
the genome.
Morgan The length of chromosome in which, on average, one recombination
event (crossover or chiasma) occurs each time a gamete is formed.
PCR Polymerase chain reaction. Technique by which a large number
of copies of a specific DNA sequence can be made very rapidly.
Polymorphism The presence of two or more alleles for a particular
gene in a population. Such genes are said to be polymorphic.
Positional cloning The cloning of a gene based upon its map position.
Probe Labelled DNA sequence used to detect complementary sequences,
for example in a genomic library or in the technique of in situ
hybridisation.
QTL Quantitative trait locus. A locus involved in the control of
a trait for which variation between individuals is usually gradual
and often controlled by many such loci. Such traits include growth
rate, fatness and litter size.
Recombination The process of genetic exchange which takes place
between a pair of chromosomes, this exchange takes place at a `crossover'.
RFLP Restriction fragment length polymorphism. A polymorphism where
different alleles vary in the length of DNA fragments produced when
DNA is cut with a restriction enzyme (an enzyme which cuts DNA only
where a specific sequence of basis is found, this sequence depending
on the actual enzyme used). RFLPs are due to changes in the DNA
sequence caused by loss or addition of a restriction site or by
deletion or insertion of DNA between two restriction sites.
Synteny Where two or more genes are on the same chromosome, such
genes are said to be syntenic. Genes far apart on the same chromosome
may be genetically unlinked (i.e. inherited independently) but still
in synteny.
VNTR Variable number of tandem repeats. Locus at which a DNA sequence
is repeated several or many times in a `head to tail' fashion (see
microsatellite and minisatellite)
Table 1. The distribution of work between the participants
Participant number
Task 1 2 3 4 5 6 7 8 9 10 11 12 13
[
]
Reference families MxL L WxP MxL MxL WxL
Homologous probes X X X X X X X X
Heterologous probes X X X X X X X
VNTRs X X X X X X X X
Microsatellites X X X X X X X X X
Protein polymorphisms X X X X
SLA polymorphisms X X
Hybrid cell lines X X X X X
In situ hybridisation X X X X X
FACS chromosome sorting X X
Cytogenetic abnormalities X X
Database development X
Statistics/Computation X X X X
[
]
Crosses: M= Meishan, L=Large White, W=Wild Boar, P=Pietrain
Participants:
1 AFRC Institute of Animal Physiology and Genetics Research, Edinburgh,
U.K.
2 AFRC Institute of Animal Physiology and Genetics Research, Cambridge,
U.K.
3 State University of Ghent, Merelbeke, Belgium.
4 The Royal Veterinary and Agricultural University, Copenhagen,
Denmark.
5 Universität Hohenheim, Stuttgart, F.R.G.
6 INRA , France.
7 State University, Utrecht, The Netherlands.
8 Wageningen Agricultural University, Wageningen, The Netherlands.
9 Norwegian College of Veterinary Medicine, Oslo , Norway.
10 Swedish University of Agricultural Sciences, Uppsala ,
Sweden.
11 National Institute of Animal Science, Foulum, Denmark.
12 Università degli Studi di Bologna, Reggio Emilia,
Italy.
13 University of Leicester, Leicester, U.K.
Figures
Figure 1. One of the best mapped regions of the porcine genome at
present is around the HAL locus, but even here there is some uncertainty
over the exact order of the loci. The HAL locus is responsible for
sensitivity to the gaseous anaesthetic halothane and also for susceptibility
of pigs to stress (porcine stress syndrome). The figure shows estimated
genetic distances between loci in Morgans. The whole region has
been physically mapped to porcine chromosome 6 using in situ
hybridisation. The HAL locus is homologous to the locus responsible
for the human condition malignant hyperthermia, a potentially lethal
reaction to anaesthetic, and provides an animal model for this disease.
The HAL locus is now known to be the same as that for a cell surface
receptor: the ryanodine receptor (RYR). The physical map positions
in man and mouse compared with the pig show that syntenic relationships
have largely been conserved: the loci found on porcine chromosome
6 tend to be together on a chromosome and in the same order in these
other species. The other loci shown are S(A O) and H which are blood
groups, glucose phosphate isomerase (GPI), phoshogluconate dehydrogenase
(PGD), alpha 1 B glycoprotein (A1BG), apolipoprotein E 1 (APOE 1),
enolase 1 (ENO1) and transforming growth factor beta 1 (TGFB1).
Figure 2. The Chinese Meishan sow: The physical appearance of this
animal alone is a good indication of its genetic distance from European
pigs.
Figure 3. The European Large White (Yorkshire): One of the most
efficient and widely used breeds world wide.
Figure 4. The European Wild Boar: Very distinct from modern European
breeds.
Figure 5. A porcine RFLP detected with a porcine albumin cDNA probe.
Four Meishan (M) and Large White (L) animals (PiGMaP reference family
founders) were sampled and the two breeds were found to be homozygous
for different alleles (Archibald et al., 1991). Each column
on this gel represents one animal. The Meishan specific allele is
band 1 on the gel and the Large White specific allele is band 3.
Bands 2 and 4 are not polymorphic and do not vary between the breeds.
Figure 6. The segregation of genes controlling pigmentation can
be seen in this litter of F2 piglets with their F1 dam. This litter
resulted from a cross between the Large White and Meishan breeds.
Figure 7. The segregation of a porcine minisatellite in a large
F1 litter resulting from a cross between a Meishan (M) sow and a
Large White (L) boar. The parents are shown at either end of the
photograph with their litter of piglets in between them. The Meishan
dam is homozygous for allele 3 and so all of the piglets inherit
this allele from their mother. The Large White sire is heterozygous
for alleles 1 and 2 and so each piglet inherits one or other of
these alleles (Signer, pers. comm.).
Figure 8. The karyotype of a hybrid cell line. These are G banded
chromosomes in a metaphase spread from a single nucleus of a pig/Chinese
hamster hybrid cell line. All Chinese hamster chromosomes are present
and are unlabelled. The labelled chromosomes are those retained
from the pig and all but one have been identified (Bosma, pers.
comm.).
Figure 9. The separation of porcine chromosomes by a dual laser
fluorescence activated cell sorter (FACS). Each of the 18 pairs
of autosomes and the X and Y chromosome is represented on the plot
by a tight scatter of points (Dixon et al., 1991. Anim.
Genet. 22, Suppl. 1:87.).
Figure 10. In situ hybridisation showing the physical mapping
of the PGD locus. This diagram represents a summary of 75 metaphase
spreads and each dot beside the porcine karyotype represents the
position of one silver grain on an autoradiograph. The large number
of silver grains on chromosome 6 indicates that PGD is on this chromosome
(Yerle et al., 1990).
Figure 11. In situ hybridisation: The physical mapping of
the rRNA genes. This shows the chromosomes from a single nucleus,
the four yellow dots mark the location of the rRNA genes on two
different pairs of porcine metaphase chromosomes (Bosma, pers. comm.).
[*]
A pamphlet prepared by Drs. Chris Haley and Alan Archibald with the
assistance of other PiGMaP participants, (translated by ???) edited
by Professor H. Bazin (Biotechnology Division, Commission of the European
Communities)
